This invention relates to fiber optic chemical sensors which indicate the presence of chemical species in solution based upon polymer swelling.
Sensors for chemical species of a variety of types are known, including many which rely on optical detection. For example, detection systems which rely on the absorbance, emission or scattering of light are known.
In recent years, sensors which incorporate fiber optics to transport light between the sample and a light source or sensor have been proposed. See Seitz, "Chemical Sensors Based on Immobilized Indicators and Fiber Optics," CRC Crit. Rev. Anal. Chem. 19, 135 (1988) and references cited therein. These systems frequently rely upon the visible or near ultraviolet spectral characteristics of a chemical species formed by reaction of an indicator reagent with the analyte. This design choice, however, gives rise to several disadvantages.
First, the wavelengths of light used in these devices are not well transmitted due to the absorption properties of the optical fibers. This can lead to a loss of sensitivity. Further, sensor stability is generally limited by the indicator. Since the sensing mechanism requires photoexcitation, photodecomposition is often a problem. It tends to become an even more serious problem when sensors are miniaturized since a relatively larger amount of light has to be directed into the indicator to maintain signals at a level where they can be measured with adequate precision. Many of the indicators are dyes originally designed for use in solution measurements on a onetime basis, an application that does not require nearly as high intrinsic stability as sensing. As a consequence many of the indicator dyes are not particularly stable.
There are also a limited number of chemical sensor applications involving optical processes other than absorption or luminescence. Chemical sensing has been based on changes in fiber transmission accompanying changes in the refractive index at the surface of the fiber core. Kawahara et al., "Development of a Novel Method for Monitoring Oils in Water", Anal. Chim. Acta, 151, 315 (1983); Sutherland et al., "Preliminary Results Obtained With a No-Label Homogeneous, Optical Immunoassay For Human Immunoglobulin G", Anal. Lett., 17, 43 (1984); and Guiliani et al., "Detection of Simple Alkanes at a Liquid-Glass Interface by Total Internal Optical Scattering", Sensors and Actuators, 6, 107 (1984). The feasibility of interferometric chemical sensing has also been demonstrated. Butler, "Optical Fiber Hydrogen Sensor", Appl. Phys. Lett; 45, 1107 (1984); and Dessy, "The Electronic Toolbox I", Anal. Chem., 57, 1188A (1985). Interaction of the analyte with an indicator phase coated around the core of a fiber constricts the fiber causing a change in the phase of the light transmitted through the fiber. This type of sensor can be extremely sensitive provided it is kept in a protected environment which minimizes the effects of vibration and temperature.
Most fiber optic chemical sensors reported to date use conventional spectroscopic instrumentation. The detector is most frequently a photomultiplier tube, although photodiodes are also common. The sources are usually a tungstenhalogen lamp, a xenon arc lamp or a laser, usually an argon ion laser. Sensor systems using continuum sources generally require a filter or a monochromator for wavelength resolution.
Light emitting diodes (LEDs) have been used in only a few sensors. Guiliani et al., "Reversible Optical Waveguide Sensors for Ammonia Vapors", Optics Lett., 8, 54 (1983); Goldfinch et al., "Solid-phase Optoelectronic Sensors for Biochemical Analysis", Anal. Biochem., 138, 430 (1984); Goldfinch et al., "A Solid-phase Optoelectronic Sensor for Serum Albumin", Anal. Biochem., 109, 216 (1980); and Freeman et al., "A Fiber-Optic Absorption Cell for Remote Determination of Copper in Industrial Electroplating Baths", Anal. Chem. Acta, 177, 121 (1985). The infrequent use of LEDs is due to the fact that they are only available at relatively long wavelengths and thus are not compatible with many indicators. An LED which emits blue light is available, but it has a very wide emission band and is considerably less intense than longer wavelength LEDs. Where applicable, LEDs are attractive sources. In addition to their low cost, long wavelength LEDs emit a relatively narrow band of light and thus do not require wavelength resolution. Furthermore, if they are maintained at a constant temperature, LEDs are extremely stable light sources after they have been "burned in" for several days. Smith et al., "High Precision Fluorimetry with a Light Emitting Diode Source", Appl. Spec., 42, 1469 (1988); and Pawliszyn, "LEDs and Laser Diodes in Schlieren Optics Methods', Rev. Sci. Instrum., 58, 245 (1987). Temperature control is important since LED emission intensities change by about 0.5% per degree Celsius.
It is an object of the present invention to provide a class of sensor devices which use a light transmission means such as fiber optics in combination with an LED or other light source to provide accurate and specific detection of a variety of chemical species in solution.